Process for the continuous production of hydrogen peroxide

ABSTRACT

A process-is described for the continuous production of alcoholic or hydro-alcoholic solutions of hydrogen peroxide in a concentration ranging from 2 to 10% by weight and their direct use in oxidation processes.  
     The process operates under high safety conditions and with a high productivity and molar selectivity towards the formation of H 2 O 2 .

[0001] The present invention relates to a process for the continuousproduction of hydrogen peroxide (H₂O₂)

[0002] More specifically, the present invention relates to a process forthe continuous production of alcoholic or hydro-alcoholic solutions ofhydrogen peroxide and their direct use in oxidation processes.

[0003] In particular, oxidation processes using hydrogen peroxide asoxidizing agent, are known in the art, in which the use of alcoholic orhydro-alcoholic solutions object of the present invention have proved tobe useful.

[0004] Examples of these processes are those using titanium silicaliteas catalysts, such as the epoxidation of olefins (EP-100,119), theammoximation of carbonyl compounds (U.S. Pat. No. 4,794,198), theoxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and thehydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783).

[0005] The industrial production of aqueous solutions of H₂O₂ by meansof a complex two-step process, is known. In this process a solution ofan anthraquinone, such as butylanthraquinone or ethylanthraquinone, inan organic medium immiscible with water is first hydrogenated and thenoxidized with air to produce H₂O₂ which is subsequently extracted inaqueous phase.

[0006] This process is expensive owing to the high investment costsnecessary for the complex production unit involved and the necessity ofseparating and disposing of the by-products generated during theoxidation phase and purifying and reintegrating the anthraquinonesolution before being re-used.

[0007] For these reasons, processes for the direct synthesis of hydrogenperoxide from H₂ and O₂ have been studied and seem attractive from atechnical and economic point of view.

[0008] These processes generally use a catalytic system consisting of anoble metal, particularly metals of the platinum group or theirmixtures, in the form of salts or as supported metals, reacting the twogases in a solvent consisting of an aqueous medium or an aqueous-organicmedium.

[0009] The embodiment of these processes however has proved to bedifficult on an industrial scale for the following reasons:

[0010] A) the use of mixtures of H₂ and O₂ in concentrations within theexplosivity range, as the mixture becomes explosive when theconcentration of H₂ exceeds a value which, depending on the pressure andconcentration of O₂, varies from 4.5 to 6% by volume:

[0011] B) even when operating outside the explosivity range of H₂—O₂mixtures, the use of high concentrations of O₂ is risky to handle andnot very compatible with the presence of flammable organic solventmediums;

[0012] C) the use in the reaction medium of high concentrations ofpromoters, for example acid, halogenated promoters and/or otheradditives, makes the catalytic system or H₂O₂ solution unstable. Thismakes it necessary to add stabilizers with costly purificationoperations of the H₂O₂ solution produced before its use;

[0013] D) low productivity and selectivity of the reaction or theproduction of H₂O₂ solutions which are too dilute for economicindustrial exploitation;

[0014] E) low stability of the catalytic system under the reactionconditions.

[0015] For example, U.S. Pat. Nos. 3,361,533, 4,009,252 and 4,661,337describe processes for the preparation of H₂O₂ which use gaseousmixtures of H₂ and O₂ which are typically included in the explosivityrange.

[0016] To avoid these safety problems, some processes use ingenious andcomplex reactor solutions.

[0017] U.S. Pat. No. 5,194,242 describes a continuous process for thepreparation of H₂O₂ in aqueous solution which comprises the use of asuitable reactor and a mixture of H₂ and O₂ in ratios with each otherwithin the explosivity range in the feeding to the reactor, but outsidethis range in the gaseous stream leaving the reactor.

[0018] U.S. Pat. No. 5,641,467 describes a continuous process for thepreparation of H₂O₂ from H₂ and O₂ which operates within the explosivityrange of H₂/O₂ mixtures using a reaction apparatus suitable forproducing a series of gas bubbles sufficiently small and sufficientlydispersed and separate from each other in the liquid reaction medium asto avoid any possible explosion in the reactor.

[0019] These processes however are complex from an industrial point ofview and their intrinsic safety is doubtful.

[0020] Numerous other processes describe, on the other hand, the use ofmixtures of H₂ and O₂ poor in hydrogen, i.e. with concentrations of H₂of less than 4-5% by volume with respect to the gaseous mixture, for thesame purpose, i.e. to avoid safety problems deriving from the use ofexplosive H₂—O₂ mixtures.

[0021] These processes, however, use extremely high concentrations ofO₂, whose use is only possible when operating in an aqueous solventmedium and they are therefore normally directed towards the productionof aqueous H₂O₂ solutions, excluding the presence of organic mediums.

[0022] For example, U.S. Pat. No. 5,500,202 describes a continuousprocess for the preparation of aqueous solutions of H₂O₂ from H₂ and O₂which operates in a “trickle bed” reactor using, in the feeding, agaseous mixture of H₂/O₂/N₂ containing 4.6-6.2% by volume of H₂ and57-62% by volume of O₂, so that the mixture leaving the reactor isoutside the explosivity range.

[0023] U.S. Pat. No. 4,279,883 describes a continuous process for thepreparation of aqueous solutions of H₂O₂ from H₂ and O₂ characterized bya particular pre-treatment of the solution and catalyst with H₂ and inwhich the mixtures of gases at the outlet of the reactor is kept withthe volume composition of 3% H₂ and 30% O₂, the remaining percentagebeing N₂.

[0024] International patent application WO 93/14025 describes a processfor the preparation of aqueous solutions of H₂O₂ from H₂ and O₂characterized by the use of particular catalysts and stabilizers of thecatalytic activity and carried out in the presence of gaseous mixtureskept outside the explosivity limits of H₂, with O₂ being fed in pureform or preferably mixed with N₂ to obtain a preferred H₂/O₂/N₂composition equal to 3.2%/86.8%/10.0% respectively by volume.

[0025] International patent application WO 92/15520 describes a processfor the preparation of aqueous solutions of H₂O₂ from H₂ and O₂characterized by the use of particular catalysts and stabilizers of thecatalytic activity and carried out in the presence of gaseous mixtureskept outside the explosivity limits of H₂, with O₂ being fed in pureform without inert gases.

[0026] European patent application EP-627,381 describes a process forthe preparation of aqueous solutions of H₂O₂ from H₂ and O₂characterized by the use of particular catalysts and carried out in thepresence of gaseous mixtures kept outside the explosivity limits of H₂in the presence of inert gases such as nitrogen so as to obtain apreferred H₂/O₂/N₂ composition equal to 3%/47%/50% respectively byvolume.

[0027] In all these processes high concentrations of O₂ are used, whichare scarcely compatible with the use of flammable organic reactionsolvent mediums.

[0028] In other cases, the use of mixtures of H₂ and O₂ outside theexplosivity range and using low concentrations of O₂ prove to have a lowreaction productivity and selectivity or to produce H₂O₂ solutions whichare too dilute for an economic industrial exploitation.

[0029] For example U.S. Pat. No. 5,082,647 describes a process for thepreparation of aqueous solutions of H₂O₂ from H₂ and O₂ characterized bythe use of particular catalysts and carried out in a trickle bedreactor, feeding a gaseous mixture containing 3% of H₂ by volume in air.In the example described, after 5 hours the solution re-circulatedthrough the reactor contained 0.3% of H₂O₂.

[0030] From what is specified above, the objective of setting up aprocess for the direct synthesis of H₂O₂ from H₂ and O₂ which can beused on an industrial scale under safety and economically advantageousconditions, does not appear to have been reached as yet.

[0031] In particular, the objective of producing stable solutions ofH₂O₂ in aqueous-organic mediums with a concentration suitable for directuse in oxidation processes, on an industrial scale and under safety andeconomically advantageous conditions, seems even more difficult toreach.

[0032] For example, U.S. Pat. Nos. 4,389,390 and 4,336,238 describe acontinuous process for the preparation of aqueous-organic solutions ofH₂O₂ from H₂ and O₂ which use a vertical tubular catalytic packedreactor operating in equicurrent of gas and liquid and using a gaseousmixture of H₂/O₂ in the feeding, containing 6.3% by volume of H₂ and95.7% by volume of O₂.

[0033] European patent applications EP-787,681 and EP-812,836, directedtowards the production of aqueous-alcoholic solutions of H₂O₂ and theiruse in the epoxidation of propylene, describe processes for thepreparation of these solutions in the presence of a halide and a metalof the platinum group, with the use of H₂/O₂/N₂ mixtures having acomposition equal to 2.4%/24%/73.6% respectively by volume, or using H₂diluted in air outside the explosivity limits, in which theconcentration of H₂O₂ in the solutions obtained ranges from 0.05% to0.39% by weight.

[0034] International patent application WO 98/16463 likewise describes aprocess directed towards the production of aqueous-alcoholic solutionscontaining at least 2.5% by weight of H₂O₂, carried out on monolithicformed catalytic bodies, which uses mixtures of H₂ and O₂, optionally inthe presence of inert gases, or mixtures of H₂ and air outside theexplosivity limits for H₂.

[0035] The illustrative examples of this document however all comprisethe use of mixtures of H₂ with pure O₂. In addition, from the examplesit can be observed that when the gas stream fed to the reactor is withinthe explosivity limits (H₂ concentration equal to 10% by volume in pureO₂), solutions of H₂O₂ are obtained at 4-7% by weight with selectivitieswith respect to H₂ ranging from 70 to 82%. Viceversa, when a gas streamis used in the feeding outside the explosivity limits, (concentration ofH₂ equal to 4% by volume in pure O₂), the concentration of H₂O₂ obtaineddoes not exceed 2.7% and the selectivity calculated with respect to H₂drops to 68%.

[0036] Therefore not even this process overcomes the problems derivingfrom the use of explosive mixtures of H₂—O₂ and from the poorcompatibility of high concentrations of O₂ with flammable organicreaction mediums.

[0037] It has now been found, according to the present invention, thatit is possible to overcome the drawbacks of the known art discussedabove, using a bimetal catalyst based on palladium and platinum asactive components, a liquid reaction medium consisting of an alcohol oran alcohol-water mixture with a prevalent alcoholic content and agaseous stream containing hydrogen, oxygen and an inert gas, in whichthe concentration of hydrogen is less than 4.5% by volume, theconcentration of oxygen is less than 21% by volume, the complement to100 being inert gas, to produce alcoholic or alcohol-aqueous solutionsof H₂O₂ at a concentration ranging from 2 to 10% by weight with a highreaction productivity and selectivity and a high stability of thecatalytic activity over a period of time.

[0038] These reaction conditions provide substantial advantages, inparticular:

[0039] (1) The possibility of carrying out the process under high safetyconditions, with respect to the handling of both hydrogen-oxygenmixtures and alcohol-oxygen mixtures. In fact, operating below 4.5% byvolume of hydrogen is well outside the explosivity zone of H₂—O₂-inertgas mixtures. Similarly, when operating below 21% by volume of O₂, risksderiving from the presence of alcohol in the gaseous phase, owing to thereduced quantity of fuel in the reaction medium, are minimized.

[0040] In addition, the concentration of H₂O₂ in the solutions producedaccording to the invention is such that there are no instabilityphenomena linked to the presence of organic solvent, phenomena whichnormally occur at much higher concentrations.

[0041] (2) The possibility of minimizing the concentrations of acid andhalogenated promoter present in the liquid reaction medium. This hasbeneficial effects on the stability of the catalytic system, on thestability of the H₂O₂ solutions obtained and on the possibility of thedirect use of the above solutions in the oxidation processes mentionedabove.

[0042] (3) The production of solutions of hydrogen peroxide with acomposition and concentration adequate for direct use and economicallyvalid in oxidation processes, generally ranging from 2% to 10% byweight.

[0043] In accordance with this, the present invention relates to aprocess for the continuous production of alcoholic or hydro-alcoholicsolutions with a prevalent alcoholic content of hydrogen peroxide, whichcomprises:

[0044] (a) feeding to a reactor, which contains a catalyst based onpalladium and platinum, heterogeneous and maintained in dispersion in aliquid reaction medium:

[0045] (i) a liquid stream consisting of an alcohol or an alcohol-watermixture with a prevalent alcoholic content containing an acid promoterand a halogenated promoter;

[0046] (ii) a gaseous stream containing hydrogen, oxygen and an inertgas, characterized in that the concentration of hydrogen is lower than4.5% by volume and the concentration of oxygen is lower than 21% byvolume, the complement to 100 being an inert gas;

[0047] (b) removing from the reactor:

[0048] (iii) a liquid stream essentially consisting of the stream

[0049] (i) and also containing hydrogen peroxide and the water producedby the reaction, characterized in that the concentration of hydrogenperoxide ranges from 2% to 10% by weight; and

[0050] (iv) a gaseous stream essentially consisting of non-reactedhydrogen and oxygen and the inert gas.

[0051] The reactor used can be any reactor suitable for operating incontinuous and conducting the reaction in a tri-phasic system such asthat described, obtaining an effective contact between the gaseousphase, liquid phase and the catalyst maintained in dispersion (so-calledslurry system).

[0052] Reactors suitable for the purpose are, for example, stirredreactors, bubble reactors, gas-lift reactors with internal or externalcirculation, as described in the art.

[0053] The reactor is kept under suitable temperature and pressureconditions. According to the process object of the present invention,the temperature normally ranges from −10° C. to 60° C., preferably from0° C. to 40° C. The pressure normally ranges from 1 to 300 bars,preferably from 40 to 150 bars.

[0054] The residence time of the liquid medium in the reactor normallyranges from 0.05 to 5 hours, preferably from 0.10 to 2 hours.

[0055] The catalyst which can be used for the purposes of the presentinvention is a heterogeneous catalyst containing palladium and platinumas active components.

[0056] In these catalysts, the palladium is normally present in aquantity ranging from 0.1 to 3% by weight and the platinum in a quantityranging from 0.01 to 1% by weight, with an atomic ratio between platinumand palladium ranging from 1/500 to 100/100.

[0057] The palladium is preferably present in a quantity ranging from0.4 to 2% by weight and the platinum in a quantity ranging from 0.02 to0.5% by weight, with an atomic ratio between platinum and palladiumranging from 1/200 to 20/100.

[0058] In addition to palladium and platinum, other metals of group VIIIor IB, such as for example, ruthenium, rhodium, iridium and gold, can bepresent as active components or promoters, in a concentration generallynot higher than that of the palladium.

[0059] The catalyst can be prepared by dispersing the active componentson an inert carrier by means of precipitation and/or impregnationstarting from precursors consisting for example of solutions of theirsalts or soluble complexes, and then reduced to the metal state by meansof thermal and/or chemical treatment with reducing substances such ashydrogen, sodium formiate, sodium citrate using preparative techniqueswell known in the art.

[0060] The inert carrier may typically consist of silica, alumina,silica-alumina, zeolites, activated carbon, and other materials wellknown in the art. Activated carbon is preferred for the preparation ofthe catalysts useful for the invention.

[0061] Activated carbons which can be used for the purposes of theinvention are selected from those of a fossil or natural origin derivingfrom wood, lignite, peat or coconut and having a surface area greaterthan 300 m²/g and which can reach 1400 m²/g, in particular those havinga surface area greater than 600 m²/g.

[0062] Preferred activated carbons are those with a low ash content.

[0063] Sulfonated activated carbons described in Italian patentapplication MI 98A01843 are also useful for the purpose.

[0064] Before supporting or impregnating the metals, the activatedcarbon can be subjected to treatment such as washing with distilledwater or treatment with acids, bases or diluted oxidizing agents, forexample acetic acid, hydrochloric acid, sodium carbonate and hydrogenperoxide.

[0065] The catalyst is normally dispersed in the reaction medium at aconcentration ranging from 0.1 to 10% by weight, preferably from 0.3 to3% by weight.

[0066] The liquid stream (i) consists of an alcohol or mixture of C₁-C₃alcohols or a mixture of these alcohols with water with a prevalentalcoholic content. A mixture with a prevalent alcoholic content means amixture containing more than 50% by weight of alcohol or mixture ofalcohols. Among C₁-C₃ alcohols methanol is preferred for the purposes ofthe present invention. Among the mixtures, a mixture of methanol andwater containing at least 70% by weight of methanol, is preferred.

[0067] The liquid stream also contains an acid promoter and halogenatedpromoter.

[0068] The acid promoter can be any substance capable of generating H⁺hydrogen ions in the liquid reaction medium and is generally selectedfrom inorganic acids such as sulfuric, phosphoric, nitric acids or fromorganic acids such as sulfonic acids. Sulfuric acid and phosphoric acidare preferred. The concentration of the acid generally ranges from 20 to1000 mg per kg of solution and preferably from 50 to 500 mg per kg ofsolution.

[0069] The halogenated promoter can be any substance capable ofgenerating halogen ions in the liquid reaction medium. Substancescapable of generating bromide ions are preferred. These substances aregenerally selected from hydrobromic acid and its salts soluble in thereaction medium, for example alkaline bromides, hydrobromic acid beingpreferred. The concentration of halogenated promoter generally rangesfrom 0.1 to 50 mg per kg of liquid medium and preferably from 1 to 10 mgper kg of liquid medium.

[0070] The gaseous stream (ii) at the inlet contains a concentration ofhydrogen of less than 4.5% by volume and a concentration of oxygen ofless than 21% by volume, the complement to 100 being an inert gas, whichis generally selected from nitrogen, helium, argon. Said gas ispreferably nitrogen.

[0071] In the gaseous stream (ii) the concentration of hydrogenpreferably ranges from 2% to 4% by volume and the concentration ofoxygen preferably ranges from 6% to 15% by volume.

[0072] The oxygen can be supplied in said stream using as raw material,pure or substantially pure oxygen, enriched air, containing for examplefrom 21 to 90% of oxygen or air, the composition of the streamthen-being brought to the desired values, defined above, by the additionof a suitable concentration of inert gas.

[0073] The liquid stream (iii) at the outlet of the reactor normally hasa concentration of hydrogen peroxide ranging from 2% to 10% by weightand preferably from 3% to 8% by weight. It also contains the acidpromoter and halogenated promoter in quantities equal to thoseintroduced together with the liquid stream fed, and water in a quantityequal to that introduced with the liquid stream fed, to which the waterobtained as reaction by-product is added. The latter normally representsan additional concentration ranging from 0.5% to 2.5% by weight.

[0074] The liquid stream (iii) is separated from the catalyst by meansof filtration techniques well known in the art, for example by the useof filter plugs situated inside the reactor or in a specificre-circulation cycle of the reaction mixture, outside the reactor. Inthis latter case the tangential filtration technique can also beconveniently used.

[0075] The liquid stream (iii) is stable to storage without thenecessity of adding stabilizing substances and is suitable for directuse in oxidation processes downstream such as the ammoximation ofcyclohexanone to cyclohexanone-oxime in the presence of ammonia and H₂O₂and the oxidation reaction with H₂O₂ of propylene to propylene oxide.

[0076] The gaseous stream (iv) at the outlet of the reactor, essentiallyconsisting of non-reacted hydrogen and oxygen and of the inert gas,generally contains a volume concentration of hydrogen equal to or lowerthan 2%, normally ranging from 0.5 to 1.5%, and a volume concentrationof oxygen generally lower than 18%, normally ranging from 6% to 12%.

[0077] In an embodiment of the process of the present invention, thegaseous stream at the outlet of the reactor is (recycled to the feedingto the reactor, after flushing from the system, the fraction necessaryfor eliminating the quantity of inert gas introduced in excess with thefeeding particularly when air is used as oxygen source. In this case,the gaseous stream (ii) fed to the reactor consists of the recycledfraction of the above stream (iv), with the addition of a quantity ofhydrogen and oxygen (as such or in the form of air or enriched air)essentially equal to that used up by the reaction and that flushed.

[0078] According to another embodiment of the process of the presentinvention, the gaseous stream (iv) leaving the reactor is fed to one ormore subsequent reactors operating analogously to the one previouslydescribed, after adding each time a quantity of hydrogen and oxygen (assuch or in the form of air or enriched air) essentially equal to thatused up by the reaction which takes place in the single reactors.

[0079] Operating under the above conditions, it is possible to producehydrogen peroxide under safety conditions with a reaction productivitynormally ranging from 30 to 200 g of H₂O₂ (expressed as 100% H₂O₂) perliter of reaction medium per hour and with a molar selectivity towardsthe formation of H₂O₂, referring to the hydrogen used up, generallyhigher than 65%.

[0080] The following examples are illustrative but do not limit thescope of the invention described.

EXAMPLE 1 Treatment of the Carrier

[0081] 50 g of activated maritime pine charcoal in powder form (CECA)and 500 ml of distilled water are charged into a 1 liter glass flask.After 2 hours at 80° C., the charcoal is filtered and washed with 500 mlof distilled water.

[0082] The carbon, still damp, is then charged into the 1 liter flaskand after adding 500 ml of a solution at 2% by weight of HCl, thetemperature is brought to 80° C. After about 2 hours, the mixture iscooled and the activated carbon is washed on a filter with distilled H₂Ountil the chlorides have been eliminated. The washed activated carbon isrecovered and dried in an oven at 120° C. for 2 hours.

EXAMPLE 2 Preparation of the Catalyst 1%Pd-0.1%Pt/C

[0083] 10 g of activated carbon treated as described in example 1, arecharged into a 0.5 liter glass flask, containing 100 ml of distilledwater and 0.32 g of Na₂CO₃. The suspension is maintained at roomtemperature (20-25° C.), under stirring, for 10 minutes.

[0084] 10 ml of an aqueous solution containing 1.0 g of a solution ofNa₂PdCl₄ at 10% by weight of Pd and 0.1 g of a solution of H₂PtCl₆ at10% by weight, are subsequently added dropwise over a period of about 10minutes.

[0085] The suspension is kept at room temperature for 10 minutes and isthen heated in a water bath for 10 minutes to 90° C. A solutioncontaining 0.85 g of sodium formiate in 10 ml of water is then added andthe stirring is continued at 90° C. for 2 hours.

[0086] After cooling to room temperature, the suspension is filtered andthe catalyst recovered is washed with distilled water until thechlorides have been eliminated and dried in an oven at 120° C. for 2hours.

EXAMPLE 3 Preparation of the Catalyst 1% Pd/C

[0087] The same procedure is used as in example 2, but using 10 ml of anaqueous solution containing 1.0 g of a solution of Na₂PdCl₄ at 10% byweight of Pd.

EXAMPLE 4 Preparation of the Catalyst 1% Pt/C

[0088] The same procedure is used as in example 2, but using 10 ml of anaqueous solution containing 1.0 g of a solution of H₂PtCl₆ at 10% byweight.

EXAMPLE 5 Synthesis of H₂O₂

[0089] A micropilot plant is used, consisting of a Hastelloy C autoclavehaving a volume of 400 ml, equipped with a thermostat-regulation system,a magnetic drag stirring-system, a regulation and control system of thepressure during the reaction, a filter for continuously removing theliquid phase containing the reaction products, a feeding system of themixture of the solvent and promoters in which the reaction takes place,a feeding system of the gaseous reagents and a series of regulation andcontrol instruments.

[0090] The reaction trend is followed by continuously analyzing thehydrogen and oxygen in the feeding and at the outlet of the reactor.

[0091] The concentration of H₂O₂ which is formed is determined in thereactor liquid effluent by titration with potassium permanganate. Theselectivity with respect to the converted hydrogen is calculated on thebasis of the concentration of H₂O₂ in the reaction effluent and on thebasis of analysis of the H₂ leaving the reactor, once the stationarystate has been reached in the reactor.

[0092] 1.2 g of catalyst prepared as described in example 2 and 150 g ofmethanol:water solution (95/5 by weight) containing 6 ppm of HBr and 300ppm of H₂SO₄ are charged into the reactor.

[0093] The autoclave is pressurized, without stirring, at 100 bars witha gaseous mixture consisting of 3.6% of H₂, 11% of O₂ and 85.4% of N₂ byvolume. The stirring is then started up to 800 revs/minute, the pressureis maintained with a continuous stream, 810 normal liters (Nl), of thesame gaseous mixture, and 300 g/hour of a methanol:water solution havingthe composition defined above, is fed at the same time. The temperatureinside the reactor is kept at 8° C. The results are indicated inTable 1. TABLE 1 Molar selectivity Hrs of reaction H₂O₂ wt % H₂O₂% 106.8 67 100 6.9 70 200 7.0 71 300 6.9 69 400 7.1 71 500 7.2 73 600 7.1 72700 7.3 74 800 7.2 73

EXAMPLE 6 (comparative) Synthesis of Hydrogen Peroxide in the Presenceof the Catalyst Pd/C

[0094] Example 5 is repeated using the catalyst prepared in example 3.

[0095] The results obtained are indicated in Table 2. TABLE 2 Molarselectivity Hrs of reaction H₂O₂ wt % H₂O₂% 5 0.7 30 10 0.6 32

EXAMPLE 7 (comparative) Synthesis of Hydrogen Peroxide in the Presenceof the Catalyst Pt/C

[0096] Example 5 is repeated using the catalyst prepared in example 4.

[0097] The results obtained are indicated in Table 3. TABLE 3 Molarselectivity Hrs of reaction H₂O₂ wt % H₂O₂% 5 1.5 20 10 1.3 24

EXAMPLE 8 (comparative)

[0098] The reaction is carried out as described in example 5, but using150 g of water instead of the hydro-alcoholic mixture. The resultsobtained are indicated in Table 4. TABLE 4 Molar selectivity Hrs ofreaction H₂O₂ wt % H₂O₂% 5 2 64 10 1.8 62 20 1.7 60 30 1.8 61

1. A process for the continuous production of alcoholic orhydro-alcoholic solutions with a prevalent alcoholic content, ofhydrogen peroxide, which comprises: (a) feeding to a reactor, whichcontains a catalyst based on palladium and platinum, heterogeneous andmaintained in dispersion in a liquid reaction medium: (i) a liquidstream consisting of an alcohol or an alcohol-water mixture with aprevalent alcoholic content containing an acid promoter and ahalogenated promoter; (ii) a gaseous stream containing hydrogen, oxygenand an inert gas, characterized in that the concentration of hydrogen islower than 4.5% by volume and the concentration of oxygen is lower than21% by volume, the complement to 100 being an inert gas; (b) removingfrom the reactor: (iii) a liquid stream consisting of the stream (i) andalso containing hydrogen peroxide and the water produced by thereaction, characterized in that the concentration of hydrogen peroxideranges from 2% to 10% by weight; and (iv) a gaseous stream essentiallyconsisting of non-reacted hydrogen and oxygen and the inert gas.
 2. Theprocess according to claim 1, wherein the catalyst contains palladium ina quantity ranging from 0.1 to 3% by weight and platinum in a quantityranging from 0.01 to 1% by weight, with an atomic ratio between platinumand palladium ranging from 1/500 to 100/100.
 3. The process according toclaim 2, wherein the catalyst contains a quantity of palladium rangingfrom 0.4 to 2% by weight and a quantity of platinum ranging from 0.02 to0.5% by weight, with an atomic ratio between platinum and palladiumranging from 1/200 to 20/100.
 4. The process according to claim 1,wherein the catalyst in addition to palladium and platinum, containsanother metal selected from those of group VIII or IB.
 5. The processaccording to claim 4, wherein the metal is ruthenium, rhodium, iridiumor gold.
 6. The process according to claim 1, wherein the catalyst isprepared by dispersing the active components on an inert carrier bymeans of precipitation and/or impregnation.
 7. The process according toclaim 6, wherein the inert carrier is selected from silica, alumina,silica-alumina, zeolites, activated carbon and activated carbonfunctionalized with sulfonic groups.
 8. The process according to claim7, wherein the carrier is an activated carbon selected from those of afossil or natural origin deriving from wood, lignite, peat or coconutand having a surface area greater than 300 m²/g.
 9. The processaccording to claim 8, wherein the carrier is an activated carbon havinga surface area which can reach a value of 1400 m²/g.
 10. The processaccording to claim 8, wherein the carrier is an activated carbon havinga surface area greater than −600 m²/g.
 11. The process according toclaim 8, wherein the activated carbon has a low ash content.
 12. Theprocess according to claim 1, wherein the catalyst is dispersed in thereaction medium at a concentration ranging from 0.1 to 10% by weight.13. The process according to claim 12, wherein the catalyst is dispersedin the reaction medium at a concentration ranging from 0.3 to 3% byweight.
 14. The process according to claim 1, wherein the liquid stream(i) consists of an alcohol or a mixture of C₁-C₃ alcohols or a mixtureof said alcohols with water with an alcoholic content higher than 50%.15. The process according to claim 14, wherein the alcohol is methanol.16. The process according to claim 14, wherein the mixture is a mixtureof methanol and water containing at least 70% by weight of methanol. 17.The process according to claim 1, wherein the halogenated promoter is asubstance capable of generating halogen ions in the reaction medium. 18.The process according to claim 17, wherein the halogenated promoter isselected from substances capable of generating bromide ions such ashydrobromic acid and its salts soluble in the reaction medium, such asalkaline bromides.
 19. The process according to claim 18, wherein thepromoter is hydrobromic acid.
 20. The process according to claim 1,wherein the concentration of halogenated promoter ranges from 0.1 to 50mg per kg of solution.
 21. The process according to claim 18, whereinthe concentration of halogenated promoter ranges from 1 to 10 mg per kgof solution.
 22. The process according to claim 1, wherein the acidpromoter is selected from substances capable of generating H⁺ hydrogenions in the reaction medium.
 23. The process according to claim 22,wherein the acid promoter is selected from inorganic acids such assulfuric, phosphoric, nitric acid or from organic acids such as sulfonicacids.
 24. The process according to claim 23, wherein the acid promoteris sulfuric acid or phosphoric acid.
 25. The process according to claim1, wherein the concentration of acid promoter ranges from 20 to 1000 mgper kg of solution.
 26. The process according to claim 25, wherein theconcentration of acid promoter ranges from 50 to 500 mg per kg ofsolution.
 27. The process according to claim 1, wherein in the gaseousstream (ii), the concentration of hydrogen ranges from 2% to 4.5% byvolume and the concentration of oxygen ranges from 6% to 15% by volume,the complement to 100 being an inert gas selected from nitrogen, he-lium and argon.
 28. The process according to claim 27, wherein the inertgas is nitrogen.
 29. The process according to claim 1, wherein in thegaseous stream (ii) the oxygen can be supplied using as raw material,pure or substantially pure oxygen, enriched air, containing from 21 to90% of oxygen or air, the composition of the stream then being broughtto the desired value by the addition of a suitable concentration ofinert gas.
 30. The process according to claim 29, wherein the liquidstream (iii) leaving the reactor has a concentration of hydrogenperoxide ranging from 3% to 8% by weight.
 31. The process according toclaim 1, wherein the liquid stream (iii) is separated from the catalystby means of filtration techniques.
 32. The process according to claim31, wherein the filtration is carried out using filter plugs situatedinside the reactor or by means of tangential filtration.
 33. The processaccording to claim 1, wherein the gaseous stream (iv) leaving thereactor, essentially consisting of non-reacted hydrogen and oxygen andinert gas, contains a volume concentration of hydrogen equal to or lowerthan 2% and a volume concentration of oxygen lower than 18%.
 34. Theprocess according to claim 33, wherein the gaseous stream (iv) leavingthe reactor contains a volume concentration of hydrogen ranging from 0.5to 1.5% and a volume concentration of oxygen ranging from 6 to 12%. 35.The process according to claim 1, wherein the gaseous stream (iv)leaving the reactor is recycled to the feeding to the reactor, afterflushing from the system the fraction necessary for eliminating thequantity of inert gas introduced in excess with the feeding and additionof H₂ and O₂ used up in the process.
 36. The process according to claim1, wherein the gaseous stream (iv) leaving the reactor is fed to one ormore subsequent reactors operating analogously to that described inclaim 1, after the addition each time of a quantity of hydrogen andoxygen essentially equal to that used up by the reaction which takesplace in the single reactors.
 37. The process according to claim 1,wherein the reaction is carried out at a temperature ranging from −10 to60° C.
 38. The process according to claim 37, wherein the temperatureranges from 0 to 40° C.
 39. The process according to claim 1, whereinthe reaction is carried out at a total pressure ranging from 1 to 300bars.
 40. The process according to claim 39, wherein the total pressureranges from 40 to 150 bars.
 41. The process according to claim 1,wherein the reactor is a reactor suitable for operating in continuousand carrying out the reaction in a triphasic system, obtaining aneffective contact between the gas phase, the liquid phase and thecatalyst held in dispersion.
 42. The process according to claim 41,wherein the reactor is selected from stirred reactors, bubble reactorsor gas-lift reactors with internal or external circulation.
 43. Theprocess according to claim 1, wherein the residence time of the liquidmedium in the reactor ranges from 0.05 to 5 hours.
 44. The processaccording to claim 43, wherein the residence time of the liquid mediumin the reactor ranges from 0.1 to 2 hours.
 45. The process according toclaim 1, wherein the liquid stream (iii) is used directly in anoxidation process of a substrate selected from olefins, aromatichydrocarbons, ammonia and carbonyl compounds using titanium silicaliteas catalysts.